Add tests for MUR subject processing

[patphongs]

The process_mur_subjects function (views.py:327-334) and its usage in mur_page needs test coverage.

Test coverage needed:

Add to fec/legal/tests/test_legal.py

class TestMurSubjectProcessing(TestCase):
def test_process_mur_subjects_with_valid_subjects()
def test_process_mur_subjects_empty_list()
def test_process_mur_subjects_missing_key()
def test_process_mur_subjects_filters_none_values()
def test_mur_page_includes_subject_list()